An AC or DC generator requires direct current to energize its magnetic field. The DC field current is obtained from a separate source called an exciter. Either rotating or static-type exciters are used for AC power generation systems. There are two types of rotating exciters: brush and brushless. The primary difference between brush and brushless exciters is the method used to transfer DC exciting current to the generator fields. Static excitation for the generator fields is provided in several forms including field-flash voltage from storage batteries and voltage from a system of solid-state components. DC generators are either separately excited or self-excited.
EXCITATION SYSTEMS in current use include direct-connected or gear-connected shaft-driven DC generators, belt-driven or separate prime mover or motor-driven DC generators, and DC supplied through static rectifiers.
Static Excitation System
As the name indicates, all the components in this type of excitation system are static. A set of rectifiers is fed from a transformer that steps down the main generator or auxiliary bus voltage. The rectifiers supply the main generator excitation current directly through slip rings and they may be controlled or uncontrolled.
An external source of DC is necessary for the initial excitation of the field windings. On engine-driven generators, the initial excitation may be obtained from the storage batteries used to start the engine or from control voltage at the switchgear.
The main advantage of the static exciter is improved response as the field current is controlled directly by the thyristor rectifier but, of course, if the generator terminal voltage is depressed too low then excitation power will be lost. It is again possible to provide the power supply from both voltage and current transformers at the generator terminals but it is doubtful whether the improved response of the static exciter over a permanent magnet brushless scheme can be justified for many small embedded generators.
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